The present invention relates to the use of casting powder for casting of steel, particularly for continuous casting, and more particularly the invention relates to improvements in such powder having practically no combined carbon.
Casting powder when used in casting has basically two purposes; first, premature solidification of the liquidous surface of the molten steel has to be prevented; second, the molten steel is protected by a layer of that powder against combining with oxygen of the surrounding air. Both objects are served by covering the otherwise free and open surface of the molten steel with a layer of material which originally has been applied as powder and remains to some extent in the powderous state but converts to some extent into a molten slag phase. Actually, it is conceivable that very little slag is formed or that practically all of the powder is converted to slag.
It follows that the layer formed by application of powder impedes or even prevents penetration of oxygen into the bath, and it impedes also radiation outflow that may cool the steel too rapidly. Clearly, the smaller the amount of outward radiation from the layer, the smaller will be the loss in thermal energy of the molten steel. For this purpose one uses casting powder -- they are commercially available -- with carbon in various states. Specifically, the carbon will be heated by the molten steel and combines with oxygen in the ambient air and which has penetrated into the powder. That effect, taken in conjunction with the formation of slag, impedes penetration of oxygen through the covering layer down to the steel underneath.
Casting powders, previously proposed to have no combined carbon, are not useable because they emit too much heat by radiation and serve, thus, insufficiently as heat shield. One could increase the amount of powder used, but it was found that control of the steel level in the mold, particularly for continuous casting becomes increasingly difficult or even impossible under conditions of an excessively thick cover on the level of steel.
Going back now to casting powder containing carbon, tests have established that some of the carbon penetrates into the steel so that the carbon content thereof is locally enriched. That in turn reduces the quality of the steel generally and results in local carbonization of the casting texture.
Another disadvantage of casting powder containing carbon is to be seen in its rather high reaction temperature. Consequently, carbon which has not reacted and powder particles which have not been molten run down along the inside of the mold, between mold sides and steel skin as it just forms. The carbon particles may react here with the skin right where still quite thin, and that in turn may result in local defects in the skin such as pores.